The update was released to protect against a backdoor Trojan horse that can allow hackers to gain remote control over your treasured iMac or MacBook.

Although there is no mention of it that we could find in Apple's release notes for Mac OS X 10.6.4, or the accompanying security bulletin, Apple has updated XProtect.plist - the rudimentary file that contains elementary signatures of a handful of Mac threats - to detect what they call HellRTS, said Graham Cluley, Sophos.

HellRTS, which Sophos products have been detecting as OSX/Pinhead-B since April, has been distributed by malicious hackers disguised as iPhoto, the photo application which ships on modern Mac computers.

If you did get infected by this malware then hackers would be able to send spam email from your Mac, take screenshots of what you are doing, access your files and clipboard and much more.

Unfortunately, many Mac users seem oblivious to security threats which can run on their computers. And that isn't helped when Apple issues an anti-malware security update like this by stealth, rather than informing the public what it has done. You have to wonder whether their keeping quiet about an anti-malware security update like this was for marketing reasons.

Apple's update to detect "HellRTS" does not offer full protection against Mac threats, Cluley said.